1. Field of the Invention
The present invention generally relates to a non-volatile memory, and more particularly to a method of accessing a non-volatile memory without being affected by coupling effect and bad column problem.
2. Description of Related Art
A flash memory is a non-volatile solid state memory device that can be electrically erased and reprogrammed. A conventional flash memory stores a single bit of information in each memory cell such that each memory cell can be programmed to assume two possible states. The conventional flash memory is thus commonly referred to as a single-bit per cell flash memory. A modern flash memory is capable of storing two or more bits of information in each memory cell such that each memory cell can be programmed to assume more than two possible states. The modern flash memory is thus commonly referred to as a multi-bit per cell flash memory.
FIG. 1 schematically shows a NAND flash memory, which is comprised of an array of floating gate memory cells 11 arranged in an n-by-m array. Word lines WL0 through WLn−1 select individual floating gate memory cells in rows. The floating gate memory cells 11 in columns are coupled drain to source to form bit lines BL0 through BLm-1, which are correspondingly coupled to sense amplifiers.
As memory storage density increases and/or more bits are stored in a single cell, capacitive coupling among the floating gate memory cells 11 increases as well. The coupling effect may be reduced, for example, by filtering high-frequency components of signals in a memory controller, or employing fault-tolerant design in a flash memory, however, at the expense of overall cost and system complexity.
A manufactured flash memory may ordinarily have some defect bit lines, usually referred to as bad columns. The bad column problem may be overcome, for example, by utilizing a replacement circuit employed in the flash memory to redirect data to a spare area in the flash memory which, however, suffers the disadvantage of high cost. The bad column problem may, alternatively, be overcome by a memory controller that skips the defect bit line while accessing the flash memory.
Conventional schemes for overcoming the bad column problem require initially storing parameters in the flash memory, based on which the memory controller then may be properly initialized for use with the flash memory. Paradoxically, the parameters that the memory controller demands for overcoming the problems caused by defect or imperfect in the flash memory may probably be initially stored in defect area of the flash memory.
A need has thus arisen to propose a novel scheme of accessing a non-volatile memory in an effective and economic manner.